Shoot Without
Noise

7.62x39 "Thumper"
Project Rifle

The
road less traveled sometimes is the more interesting journey.

You
are about to enter a region of the reloading world where no official
published data exists, where truths, half-truths and old shooter's
tales are inexorably entwined. The entire miscellany of legend
and fact is bound up in the threads of history, which must be
carefully unwrapped in order to get at the truth. There is a
general fascination in the firearms world with the unusual and
the exotic. This tale involves both.

Ever since firearms became practical
tools, methods have been explored to minimize or eliminate the
noise associated with muzzle blast. While it is not possible
to completely eliminate (silence) all the various noises involved
in the firing process, it is possible to greatly reduce the magnitude
of the muzzle signature.

The earliest methods of noise
reduction were developed shortly after the introduction of the
metallic cartridge with major innovations in quiet shooting being
established during the period encompassing WWI and WWII. One
sound damping method is widely known and another has been relegated
to the sidelines of history as a firearm oddity. It is the second,
less known, method that will be investigated here.

The well-known method of reducing
muzzle signature is, of course, the Silencer or more correctly
the Suppressor, invented by Hiram Maxim (right) just prior to
WWI. However suppressors did not see widespread use until their
adoption by various Special Forces during WWII.

Silent shooting method number
two was pioneered by the Russians during WWII and then afterwards
kept alive and further developed by the Finns. Ironically, it
has been Finland, with no restrictions on the ownership of Suppressors
that has done the most to further the development of subsonic
and silent shooting technology.

The basic theory of subsonic
technology is the use of small charges of fast burning pistol
powder to propel heavy projectiles at velocities below the speed
of sound from otherwise conventional rifles. The small powder
charge is entirely consumed within the bore resulting in an extremely
reduced muzzle report while the large, heavy projectile yields
an effective range measured in hundreds of yards.

Why shoot subsonic? A quiet rifle
is less intimidating to a novice making marksmanship instruction
easier and friendlier. Quiet shooting can improve relations with
the neighbors and reduce the damage to hunters' ears. Reduced
muzzle signatures are viewed as being less dangerous and more
humane for tactical, law enforcement and conservation purposes.

PROJECT RIFLE

Maxim Silencer

Unable to find much in the way of
factual or recent information through open sources, I decided
to build a rifle specifically to explore the world of subsonic
shooting. It started with an unused Mauser 98K action. The project
was specifically limited to readily available, off-the-shelf
components. This simple constraint would drastically reduce costs
by allowing the use of an existing cartridge case, a commonly
available barrel and off-the-shelf dies and bullets, making the
idea accessible to anyone. Once the basic constraints for the
project had evolved a calibre and cartridge had to be decided
on.

RESEARCH

Perusing firearms history books
I discovered the .45 caliber WWII DeLisle Carbine. The inherently
subsonic .45
ACP throws a big heavy bullet but it has the aerodynamic
shape of a brick. Modern versions include the integrally suppressed
Destroyer Carbine (9mm)
and Ruger 77/44 (.44
Magnum), which also use large bus-shaped bullets. I was looking
for something a little more aerodynamic to stretch the rifle's
effective range beyond 200 yards.

Limiting velocity to less than
1000 fps meant that bullet weight had to be substantial to carry
hitting and penetration power out to longer ranges. The .50 cal
and .338 cal fit these parameters but bullets, barrels and brass
become a costly endeavor. Going this route would also mean a
custom wildcat cartridge, which was not within the project parameters.

Subsonic loading techniques actually
began in WWII with the Russian Moisin Nagant rifle's 7.62x54R
cartridge. There are more different .30 cal cartridges than any
other caliber giving the best chance of finding a suitable case.
The .30 cal also has the benefit of having large number of different
bullet weights and styles to choose from and barrels are easy
to get.

With the caliber chosen a cartridge
case and some reference load data was still needed. There is
a fair amount of basic information on subsonic loads to be found
on the Internet, however, in our litigious world, there is little
published load data for specific cartridges. Where data was published,
it was almost always for use with lightweight bullets or cartridges
much larger than I wanted to work with. It seems very few people
have experimented with heavy weight, subsonic bullets fired from
medium sized cartridges.

Trolling through load manuals
and cartridge specifications I made a startling discovery. The
7.62x39
case (case
image) volume is almost identical to that of the .44
Magnum with a bullet seated. Additionally .44 Magnum load
data included bullet weights from 180 grains to 220 grains, within
the range I wanted to explore. Finally here was a suitable .30
calibre cartridge and load data that could be adapted to it.
Note: Only very experienced handloaders should attempt to develop
new load data.

To stay within the project's
"off the shelf" limitation, a factory 1:10 twist .30-06
take-off barrel was chosen. This twist rate will stabilize a
220 grain round nose bullet at 950 feet per second. A minimum
length 18 inch barrel was specified to give the rifle lightweight
and fast handling characteristics. It also proves the capacity
of the rifle to be easily adapted to an integral suppressor without
excessive over-all-length and it is the shortest barrel length
allowed by Canadian law.

The benefit of the 7.62x39 case
is that it can be easily adapted to a number of platforms and
will function through a semi-automatic rifle like the AR15/M16.
The 7.62x39 uses a .311 calibre but it can be easily adapted
to the slightly smaller but much more common .308 calibre. The
.308 bore allows a wide variety of bullet weights and styles
to perform various jobs. .311 sized bullets can be shot through
a .308 bore without a problem. In these respects the subsonic
7.62x39 (.308) is much more versatile than others.

Exotic rifle systems made primarily
of Unobtanium are useless to all but the largest and best-funded
law enforcement agencies. The use of exotic cartridges also means
that policing up brass becomes critical to avoid revealing the
user's identity. The 7.62x39, being in use with more than half
the world's armies and a huge majority of the world's terrorists
and freedom fighters, makes empty brass an untraceable entity.

SUBSONIC AMMUNITION

Subsonic 7.62x39

Boat-tail bullet seated backward
for better terminal effects.

When I started this project four
years ago, you couldn't just pop down to the local gunstore and
buy some subsonic ammunition. Things have changed a bit in the
past few years with a number of commercial subsonic loadings
available in military calibers.

Even with its current popularity
and availability, there is a lot of work to be accomplished in
the world of subsonic ammunition. Most commercial subsonics use
off the shelf, conventional bullets that are limited by the twist
rates that will stabilize them and the amount of terminal performance
they can deliver in soft targets. There is little if any subsonic
rifle reloading data available in manuals or on the internet.

There is still a role for experimentation
and handloaded ammunition before the big companies take over
and every hardware store from here to there has subsonics on
the shelf.

Loading subsonic ammunition is
not nearly as simple as metering a few grains of pistol powder
into the case and seating a big, heavy bullet. Developing new
load data where none exists can be a bit nerve wracking. Potential
dangers are minimized through specific techniques involving case,
bullet and bore preparation as well as powder choice for burning
speed and load density. Go here for a more detailed discussion
of Subsonic Loading techniques.

Initial subsonic test loads involved
several different pistol powders pushing 200 grain bullets to
find safe, benchmark data. Loads were started high and worked
down a half grain at a time with load density and muzzle velocity
monitored for each round. Ultimately it was possible to safely
reduce muzzle velocity to 875 fps. Armed with this data, loads
were increased to 950 - 1000 feet per second with confidence
that the working ammunition was safe.

Velocity should be 50-100 fps less
than the speed of sound or about 950 fps. This puts velocity
below the transonic range where the supersonic crack begins to
be generated. The sonic crack is not generated at exactly the
speed of sound. The sound increases in volume across the transonic
range starting about 92-93 percent of the speed of sound based
upon research by Al Paulson. Also keep in mind that a sound suppressor
(silencer) may create about 30 fps of freebore boost, which in
turn argues for lowering projectile velocity an additional 30-50
fps. At about 50 fps above the speed of sound the sonic crack
reaches maximum volume.

SUBSONIC BULLET CHOICE

Subsonic loads are a constant
battle between opposing forces. With velocity fixed below 1000
fps, terminal energy can only be increased by increasing bullet
weight. Increased weight eventually runs smack dab into bullet
stability problems. The longer and heavier the bullet, the more
difficult it is to stabilize. We need to walk the line between
bullet weight and stability.

Bullet weight can be increased
without increasing length by changing the shape from pointed
to round nosed or even a full wadcutter design. However each
of these changes has it's own limitations. Wadcutter shapes,
with their square nose, cause feeding problems in rifle actions
and round nosed bullets do not perform terminally as well as
we would like them to.

In flesh, a sharply pointed subsonic
bullet will simply push tissue aside without causing much terminal
damage. A blunt bullet nose with a sharp edge causes more terminal
damage because it cuts tissue as it passes, leaving the maximum
permanent cavity and wounding potential. Jacketed hollow point
bullets are designed to operate within a narrow velocity range
and most will not expand reliably at low velocity.

From observation, impact energy
doesn't appear to be as important to a subsonic bullet as it
is for conventional velocity bullets. The subsonic bullet relies
on its shape to cut a wound path. Lighter weight subsonic bullets
have more than enough energy to traverse a tactical target due
to the lack of impact deformation. Lighter bullets offer a flatter
trajectory, which is quite important.

The ideal bullet design and weight
depends on the operating parameters for the rifle but generally
it can be advantageous to sacrifice some weight and energy to
gain stability and trajectory. In a .30 calibre system, a 168
grain BTHP flying backwards is probably the best choice. This
round will feed reliably in a bolt action and give the best terminal
performance while allowing accurate shots to 200 yards.

Coincidentally the BTHP flying
backwards is the shape that is the quietest aerodynamically.
Studies have shown that a teardrop shaped projectile produces
the least flight noise. A BTHP bullet travelling backwards is
as close to a teardrop shape as can be had in a commercially
available bullet. A subsonic projectile makes a distinct flight
noise that is easy to track back to the shooter. The only fly
in the ointment is that subsonic bullets are most accurate with
a flat base so it can be difficult to get backward loaded HPBT's
to shoot accurately.

LOAD TESTING

Subsonic Oddities

Shoot the same bullet forwards
and backwards and the first thing you'll notice is a blunt nose
reduces the ballistic coefficient causing a more pronounced trajectory.
Velocity comparisons at the muzzle and downrange demonstrate
that subsonic flight produces a high ballistic coefficient (about
0.500), much higher than the identical projectile would have
at conventional velocity. This varies somewhat with the weight
and shape of the bullet.

One of the really weird phenomena
to contemplate, is that a subsonic bullet can be unstable flying
forward yet be stable flying backward. This has to do with the
relationship between the center of gravity (CG) and the center
of friction (CF). In long pointed bullets the CG is often in
the rear half of the projectile. If the CG is in front of the
CF (as it would be in a bullet flying backwards) the bullet will
be stable for the same reasons a badminton bird is stable, the
weight at the front drags the lighter tail along behind it.

Another subsonic oddity involves
a bullet that is stable at 100 yards but begins to show increasing
instability with distance, culminating in sideways impacts at
around 200 yards. Conventional theory states that a bullet, which
is stable out of the muzzle, will continue to be stable throughout
its flight, with stability increasing because forward velocity
decreases faster than rotational velocity. Interestingly unstable
projectiles maintained reasonable accuracy reasonably with groups
ranging from 1.85 MOA to 2.5 MOA. This also flies in the face
of conventional wisdom.

Spark photograph of a subsonic
bullet (above) shows lack of supersonic shockwave as is visible
on the image of the supersonic bullet (below)

Initial tests included a number
of different bullet weights and shapes ranging from 150 grain
FMJ-BT to 220 grain round nose. The two bullets eventually settled
on were the 150 grain FMJ-BT and the 180 grain round nose.

The lighter weight boat-tail
is stable in flight both forwards and backwards and surplus .30
cal pulled bullets are cheap to experiment with. The heavier
round nose bullet is the most stabilized of all bullets and offers
more weight and downrange energy. It has an additional benefit
of flying an almost identical trajectory to the 150 grain FMJ-BT
in the forward configuration.

Experimentation with subsonic
projectiles shows there is a large grey area between a stable
bullet and an unstable bullet and the parameters governing stability
at conventional velocities do not apply. A lot of time was spent
testing different bullet sizes and shapes to determine stability.
Generally lighter weights and round nose shapes gave the greatest
stability both in flight and terminally. Go here for more information
on bullet stability.

Investigation into the issue
of projectile stability suggests that subsonic projectiles require
a much greater degree of stabilization than supersonic projectiles.
Ballistic stability calculators show that a stability factor
of 1.0 - 1.3 is required for supersonic flight stability. Experimental
data shows that subsonic .30 calibre projectiles are not flight
stable with a stability factor of less than 2.0. Stability depends
greatly on the shape and fore-aft balance of the projectile.
A bullet with stability as high as 2.65 was found not to be stable
yet a specially designed subsonic bullet with a stability factor
of only 1.36 was stable.

Subsonic projectiles with a stability
factor of about 2.0 - 2.2 can be stable in flight but display
terminal instability. During testing heavy plastic and steel
drums were used as target stands. Examination of the drums revealed
that marginally stable subsonic bullets usually yawed through
90 degrees within 24 inches after impact, to exit the drum completely
sideways. Amazingly enough the sideways bullets traveled in a
reasonably straight line for a short distance after exiting the
drum.

The heavy plastic construction
of the drums represented a difficult test of a subsonic bullet's
ability to penetrate. None of the bullets were contained by the
drum. Unlike steel, plastic can give and stretch before it breaks.
The side of the drum was about 1/8 inch thick with the plastic
being of very solid construction. The entry holes clearly bore
the rifling marks engraved on the bullet. The exit holes were
much more impressive because the bullet had yawed completely
sideways to present the maximum possible striking area to the
drum side. With no sharp edges to help cut through the hard plastic,
the bullet's retained energy simply bullied it through the thick
plastic.

At every shot a shower of concrete
dust erupted from behind the drum. Not only did the subsonic
bullet have the power to penetrate both sides of the drum but
it was doing its best to beat up the concrete blocks behind as
well. This suggests that subsonic rounds could easily penetrate
vehicle bodies and other light cover while retaining the ability
to fly straight.

Getting accuracy out of subsonic
loads turned out to be a challenge. Accuracy is not comparable
to conventional ammunition and it tends to get worse with distance.

The practical limit of subsonic
range seems to be about 200 yards. It is certainly possible to
shoot further but range and wind estimations become absolutely
critical. The artillery like trajectory of means that even a
minor range error could result in a miss. For example, from a
100 yard zero, bullet drop at 150 yards was 16" and drop
at 190 yards was 39". At 100 yards a 180 grain subsonic
bullet is falling almost 6.5 inches every 25 yards. Windage estimation
is equally important because the slow moving bullets leave a
lot of time for the wind to do its damage.

FINAL THOUGHTS

The 7.62x39 subsonic rifle has
proven to be amazingly versatile. It can operate with conventional
ammunition, specialty high
velocity/low signature ammunition and subsonic
ammunition. Bullet styles and weights can range from 110
grain polycarbonate tipped to 220 grain solids to fill just about
any mission essential need.

Subsonic 180 grain and 200 grain
loads produce unsuppressed sound pressure levels (SPL's) of 152
dB, which is the same as an unsuppressed .22 Rimfire pistol.
The huge noise reduction factor (approx 12 dB) over conventional
ammunition (roughly 60% less noise) is bound to expand the tactical
applications of the subsonic rifle. The addition of a suppressor
should bring the muzzle signature down to the arena of a suppressed
.22 rimfire pistol (approximately 130 dB). Sound Test Data courtesy of Al Paulson.

Windows and glass present a conundrum
to the sniper. The target can be seen clearly and glass offers
little protection from even a thrown rock, yet it presents an
incredibly hard surface for a supersonic rifle bullet, especially
for those designed to expand upon impact. The trend of law enforcement
agencies utilizing solid copper bullets for glass penetration
has not at all addressed the potential problems of deflection
and secondary missiles.

Low velocity projectiles reduce
the number and speed of secondary missiles created by the glass
impact in addition to reducing impact deformation and deflection.
Even if a subsonic bullet is deformed or destabilized by the
initial impact, testing has shown that destabilized subsonic
bullets continue to fly in a reasonably straight line.

The primary tactical use for
a large caliber, subsonic projectile in Law Enforcement would
be to engage targets through glass or other "penetrable"
cover such as vehicle bodies. Supersonic projectiles can be deflected
or even disintegrate as a result of the high velocity impact
with glass or sheet metal, missing the intended target completely.
This effect is magnified by the use of frangible projectiles
and was demonstrated dramatically in one highly publicized police
sniper shooting where the hostage taker was missed completely
by the bullet and went on to shoot several of the hostages as
a result. The news video of this incident was broadcast around
the world, the sort of publicity that most police forces would
surely like to avoid.

7.62x39 "Thumper"
during field testing

Author's Note: During the preparation
of this article a very real negative stigma attached to the loading
of quiet ammunition by civilians was revealed. Ironically most
handloaders have no issue with building bigger and faster cartridges.
Which is more lethal; the subsonic rifle with a limited range
and non-expanding bullet or the large magnum rifle able to engage
targets at a mile with polymer tipped bullets that disintegrate
explosively? Subsonic loads are another facet of reloading that
do not render a firearm any more or less deadly.